Often the display is normally switched on in the morning and switched off in the evening after work. This means that the display is on throughout the day, consuming energy, although it may be in efficient use only for a short period of the day. At some places of work, the display is not switched off at all between working days. This being the case, many different solutions have been developed to reduce the power consumption of display units.
One known solution, which is more aimed at increasing the lifetime of the cathode-ray tube than energy saving, is to use a program in the computer system which blanks the screen when the system has not been used for a certain period of time. The main purpose of this function is to prevent figures that remain unchanged for a long period of time from burning into the fluorescent material of the cathode-ray tube. At its simplest, a function like this just shuts off the video signal from the display driver.
A substantial reduction in energy consumption can be achieved with a display structure in which the display is turned off after a certain period from the moment when the system was last used. This can be implemented, for example, with a circuit which detects the absence of a video signal and cuts power input to the most power-consuming parts of the display unit for the time when the signal is absent. With a structure of this kind, it is possible to reduce power consumption in the energy saving mode to approximately 5-8 W.
The Video Electronics Standards Association (VESA) has specified the Display Power Management Signalling system (DPMS, which is based on the monitor synchronizing signals). In the DPMS system, the operation of the display is divided into four different modes:
the ON mode, which corresponds to the normal operation of the display; PA1 the Stand-By mode, in which the display screen is blanked, for example, and thus a small reduction in power consumption is achieved; PA1 the Suspend mode, in which a substantial part of the display operations are switched off, and PA1 the OFF mode, in which nearly all display operations are switched off to the extent that it is possible. PA1 power is fed from the mains supply to the secondary circuit of the power source by means of a main power source, PA1 power input from the mains to the secondary circuit of the power source by means of passive components, such as capacitors, and PA1 use of a separate power source.
Power consumption is the lowest in the Suspend and OFF modes.
In the DPMS system, the desired mode of operation of the display unit is implemented by means of vertical and horizontal synchronizing pulses. The display must be able to interpret the desired mode of operation from the levels of the vertical and horizontal synchronizing pulses and be able to change to the level of operation specified by the synchronizing pulses. The states of the synchronizing pulses that correspond to various modes of operation are given in the table below.
______________________________________ Horizontal Vertical synchronizing synchronizing Power saving mode pulses pulses ______________________________________ ON mode Yes Yes Stand-By mode No Yes Suspend mode Yes No OFF mode No No ______________________________________
In the table, "Yes" means that the frequency and pulse ratio of the incoming signal are over the threshold value defined in the DPMS system. Correspondingly, "No" means that the frequency and pulse ratio of the incoming signal are below the threshold value.
In a solution according to the DPMS system, the circuit that interprets the synchronizing signals must be able to measure the frequency and pulse ratio of the synchronizing pulses, when required. The solution must also be capable of verifying the current situation in order to avoid errors in a situation in which the display driver changes the resolution, for example. In addition, energy is needed for interpreting the synchronizing pulses, and due to the characteristics of the technique used, this energy cannot be taken from the display driver. A common solution for interpreting the synchronizing pulses and controlling the display is to use the microprocessor of the display.
One solution that is commonly used with a DPMS system is the so-called Soft Power switch. In this case, the equipment does not have an actual main power switch for switching off the device, but it has been replaced by a switch connected to the processor. By this switch, the device can be switched to the OFF mode regardless of the state of the synchronizing signals. To the user, the OFF mode looks like the device had been switched off from the main power switch.
There are now three types of structures available for power input to the secondary circuit of the power source during the extreme power saving mode, namely the OFF mode:
In the first modification of the solution using a main power source, the secondary circuit of the power source has switching means, by which power-consuming parts of the system are switched off. The switching off can be carried out by cutting either the control signals of the parts or the operating voltages of the parts. In the suspend mode, power-intensive blocks are switched off. In the OFF mode, all other blocks except the processor or a corresponding circuit that interprets the synchronizing signals and controls the device are switched off. In a solution like this, the power source of the device operates continuously and produces continuous, stabilized operating voltages for the secondary circuit. This solution has the advantage of simple construction, but the efficiency of the power source remains low. In addition, a large number of switching means are required, if there is a large number of operating voltages in the system.
In another modification of the solution using a main power source, the operating voltages of the secondary circuit are stabilized in the OFF mode to a substantially lower level than in normal operation, whereby the operation of circuits loading the secondary circuit is prevented, and the power consumption of the circuits falls. In practice, this generally takes place so that a high, stabilized operating voltage of a secondary circuit, such as 150 volts, from which other operating voltages of the secondary circuit are formed, is stabilized to a level of approx. 8 volts. The operating voltage required by the processor is taken from the lowered operating voltage (for example, +5 V) by means of a regulator. In this modification, the stabilization of voltages to a lower level than normally corresponds to the function of the switches in the first modification. In addition to the means for lowering the voltage, a switch is needed to switch the lowered secondary voltage as the operating voltage of the processor during the power saving mode. This solution still has the advantage of being simple, and the efficiency of the power source is also somewhat improved, because the voltage amplitudes generated in the power source are smaller compared to the first solution. The complexity of the switching is not dependent on the number of operating voltages, because all the operating voltages are reduced at the same time.
In a third modification using a main power source, the operation of the power source is not continuous, but energy is fed to the secondary circuit in pulses. Thus the efficiency of the power source is still somewhat improved. In a system like this, the operating voltages of the secondary are not stable during the power saving mode, but they include oscillation at the frequency of the pulses. However, the processor receives a sufficient operating voltage to continue its operation without interruption.
Another basic solution uses passive components, such as capacitors, for transmitting power from the mains to the secondary circuit. The capacitive current of the capacitors is rectified and filtered into a direct current in the secondary circuit. In order that the power thus obtained would be sufficient for ordinary processors, the capacitance of the capacitors must be high, whereby their physical size and costs also become high.
In systems where a separate power source is used, the second power source is used when the main power supply is switched off. The separate power source is optimized for low powers, and it normally only feeds the power to the processor. The advantages of this system include reliable operation and good efficiency of the power source. However, the need of two power sources is a disadvantage, which increases the component costs.